The present invention relates generally to an apparatus for synchronization of digital video and, more particularly, to a digital video synchronizer providing parallel and coordinated synchronization paths for decoded and undecoded video.
For the processing of video signals to proceed without visible flaws, all signals must be precisely synchronized. The most important feature of this synchronization process is that it must be transparent, that is to say that is must change the signal as little as possible. For many years, this task has been accomplished by digital video synchronizers that digitized an analog composite video signal, wrote the digitized composite video signal into a memory using a clock that was locked to the input video signal, read the digitized composite video signal out of the memory using a clock that was locked to the station reference synchronizing generator, and converted the digitized composite video signal back to analog to provide an analog signal that was synchronized in time to the video studio or broadcast station.
With the advent of digital video and its increasingly common use in television studios, current video synchronizers are constricted primarily as decoders to create a digital video signal from the composite analog signal, and only secondarily as synchronizers to bring the signal into time synchronization with the other signals in the TV studio. These video synchronizer devices digitize the input composite video signal and separate it into its luminance and chrominance components. The chroma is then demodulated to R-Y and B-Y components to provide a digital video signal. This digital video signal is then synchronized to bring it into time synchronization with the studio signals. When such devices provide an analog output, they do so by using an encoder to convert the digital component signal back to an analog composite signal to feed the analog equipment in the studio.
Unfortunately, this process is not nearly as transparent as the previous process. The video decoder/synchronizers that generate an analog output using an encoder and a digital-to-analog converter (DAC) after the synchronizer create errors in the composite video output. The errors are caused by the problems inherent in the operation of a comb filter and the decoding and encoding process. These errors cause visible defects in the output signal, particularly after several generations of processing.
Today's video studios and broadcast stations are in a transition period where most studios are equipped with both composite analog equipment and digital equipment. The true need in this situation is to have a synchronizer that can operate both ways—without decoding the signal so there is maximum transparency and with decoding so that there can be a digital output.